An ion guide may be utilized to transmit ions in various types of ion processing devices, one example being a mass spectrometer (MS). The theory, design and operation of various types of mass spectrometers are well-known to persons skilled in the art and thus need not be detailed in the present disclosure. A commonly employed ion guide is based on a multipole electrode structure, which is typically an RF-only electrode structure in which the ions passing through the ion guide are subjected to a two-dimensional RF trapping field that focuses the ions along an axial path through the electrode structure. In a curved ion guide the ion axis along which the ions pass is a curved path rather than a straight path. The curved ion guide is often desirable for implementation in ion processors such as mass spectrometers because it can improve the sensitivity and robustness of the mass spectrometer. A primary advantage of the curved ion guide in such a context is that it provides a line-of-sight separation of the neutral noise, large droplet noise, or photons from the ions, thereby preventing the neutral components from reaching the more sensitive parts of the ion optics and ion detector. Moreover, the curved ion guide enables the folding or turning of ion paths and allows smaller footprints in the associated instruments.
As appreciated by persons skilled in the art, in a curved ion guide the ions are transmitted around a curved ion path through oscillations inside the radial trapping field provided by the RF voltage applied on the rods (i.e., electrodes) of the ion guide. In the absence of the RF field, the ions would move straight and eventually hit the ion guide rods. Therefore, in the curved ion guide the ions need to experience a certain minimum amount of RF restoring force during their flight before they move too close to the ion guide rods and become unstable. When the ion guide transmits one mass at a time, the best performance is obtained when the RF voltage is scanned as a function of mass to optimize transmission. However, it is often desirable to run ions at higher energy and/or transmit ions of multiple different masses (mass-to-charge, or m/z, ratio) simultaneously. In such cases, some of the ions cannot have optimal transmission conditions and they are lost, leading to less than optimal instrument sensitivity.
Accordingly, there continues to be a need for improved curved ion guides, including ion guides capable of transmitting ions at high levels of kinetic energy and simultaneously transmitting ions of multiple masses while maintaining optimized ion transmission conditions.